src: drop dead and duplicate function declarations

Four declarations named functions that have no definition anywhere, so
they were never exported (absent from libhttrack.so) and any caller
would fail to link: htswrap_set_userdef and htswrap_get_userdef (the
live path is the CHAIN_FUNCTION ARGUMENT with CALLBACKARG_USERDEF),
antislash_unescaped, and the internal liens_record. escape_remove_control
was additionally declared twice in httrack-library.h; the documented
declaration stays, the bare duplicate goes.

Header-only cleanup. The exported symbol set is unchanged (verified with
nm -D), so this is not an ABI break and needs no soname bump.

Found while documenting the public API (#382).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Signed-off-by: Xavier Roche <roche@httrack.com>

src/htscore.h

@@ -369,10 +369,6 @@ char *readfile_or(const char *fil, const char *defaultdata);
 void check_rate(TStamp stat_timestart, int maxrate);
 #endif
 
-// links
-int liens_record(char *adr, char *fil, char *save, char *former_adr,
-                 char *former_fil, char *codebase);
-
 /* Backing (download-slot) scheduler. Operate on the back[] ring (struct_back).
    Not thread-safe; call from the single crawl loop. */
 

src/httrack-library.h

@@ -254,15 +254,6 @@ HTSEXT_API int htswrap_add(httrackp * opt, const char *name, void *fct);
    or 0 if none or unknown. */
 HTSEXT_API uintptr_t htswrap_read(httrackp * opt, const char *name);
 
-/** @warning No implementation is linked into the library; calling this fails to
-    link. For per-callback user data use the CHAIN_FUNCTION() ARGUMENT and
-    CALLBACKARG_USERDEF() instead. */
-HTSEXT_API int htswrap_set_userdef(httrackp * opt, void *userdef);
-
-/** @warning No implementation is linked into the library; calling this fails to
-    link. Read per-callback user data with CALLBACKARG_USERDEF() instead. */
-HTSEXT_API void *htswrap_get_userdef(httrackp * opt);
-
 /* Internal library allocators, if a different libc is being used by the client */
 /** strdup() through the library allocator. Returns a heap copy freed with
     hts_free(), or NULL on failure. */
@@ -584,12 +575,6 @@ HTSEXT_API char *unescape_http(char *const catbuff, const size_t size, const cha
     space. Returns @p catbuff. */
 HTSEXT_API char *unescape_http_unharm(char *const catbuff, const size_t size, const char *s, const int no_high);
 
-/** @warning No implementation is linked into the library; calling this fails to
-    link. */
-HTSEXT_API char *antislash_unescaped(char *catbuff, const char *s);
-
-HTSEXT_API void escape_remove_control(char *s);
-
 /** Determine the MIME type of local file name @p fil into @p s (capacity
     @p ssize): user --assume rules, then ".html", then the built-in extension
     table. @p flag != 0 forces a fallback type. @return 1 if a type was written,